Method for a general near optimal turbo code trellis termination

ABSTRACT

A method of terminating two or more constituent encoders of a turbo encoder employing a turbo code, comprising the step of: generating tail input bits at each of two or more constituent encoders, including deriving the tail input bits from each of the two or more constituent encoders separately from a contents of shift registers within each of the two or more constituent encoders, after an encoding of information bits by the two or more constituent encoders; puncturing one or more tail output bits such that 1/R output tail bits are transmitted for each of a plurality of trellis branches, wherein R is a turbo code rate employed by the turbo encoder during an information bit transmission. In yet another variation, the step of puncturing the tail output bits further comprises the step of: transmitting, during trellis termination, the tail output bits, only if they are sent from an output branch of one of the two or more constituent encoders that are used during information bit transmission.

This applications claims benefit of U.S. Provisional Application Ser.No. 60/098,111, filed Aug. 27, 1998.

BACKGROUND OF THE INVENTION

The present invention relates to methods for terminating both of theconstituent encoders of a turbo code and developing puncturing patternsapplicable at a trellis termination stage that ensures the same numberof transmitted bits for each trellis stage during the information bittransmission and trellis termination stages.

The process of forward and reverse link turbo encoding and decoding,specifically for Code Division Multiple Access (CDMA) communicationschannels, is thoroughly described in copending U.S. patent applicationSer. No. 09/248,338 (Attorney Docket No. PD-980024) of Eroz, et al., forSETS OF RATE-COMPATIBLE UNIVERSAL TURBO CODES NEARLY OPTIMIZED OVERVARIOUS RATES AND INTERLEAVER DELAYS, filed Feb. 11, 1999, and copendingU.S. patent application Ser. No. 09/235,582, (Attorney Docket No.PD-980163) of Eroz, et al., for FORWARD ERROR CORRECTION SCHEME FOR DATACHANNELS USING UNIVERSAL TURBO CODE, filed Jan. 22, 1999, both of whichare incorporated herein by reference.

In a convolutional encoding scheme, tail bits are inserted afterinformation bits, to zero out all shift registers of an encoder. Forfeed forward encoders, tail bits are equal to zero. For feedbackencoders the value of tail bits depend on the contents of the shiftregister current values.

A turbo encoder consists of a parallel concatenation of two (2) or morerecursive (feedback) convolutional encoders. Because each constituentencoder processes the information bits in a different order due to aturbo interleaver, it is not possible to terminate all constituentencoders by the same tail bits.

A trellis termination method general enough to be used for a set ofturbo codes with different code rates as in the third generation CDMAsystems is desirable. Included in the desirable general method is amethod of puncturing tail bit sequences.

SUMMARY OF THE INVENTION

The present invention advantageously addresses the needs above as wellas other needs by providing a method and apparatus for a general TurboCode trellis termination which may be employed when a turbo encoderoperates within a wide range of turbo code rates when transmittinginformation bits.

In its most general form, the invention can be characterized as a methodof terminating two or more constituent encoders of a turbo encoder. Themethod comprises the steps of: generating tail input bits at each of twoor more constituent encoders, including the step of deriving the tailinput bits from each of the two or more constituent encoders separatelyfrom the contents of shift registers within each of the two or moreconstituent encoders, after an encoding of information bits by the twoor more constituent encoders; and puncturing one or more tail outputbits such that 1/R tail output bits are transmitted for each of aplurality of trellis stages, wherein R is a turbo code rate employed bythe turbo encoder during the information bit transmission.

In yet another variation, the step of puncturing the one or more tailoutput bits further comprises the step of: transmitting, during trellistermination, the tail output bits only if they are sent from an outputbranch of one of the two or more constituent encoders that is usedduring information bit transmission.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the presentinvention will be more apparent from the following more particulardescription thereof, presented in conjunction with the followingdrawings wherein:

FIG. 1 is a block diagram of a turbo encoder with interleaved bitsentering a second encoder, for use in accordance with one embodiment ofthe present invention.

Corresponding reference characters indicate corresponding componentsthroughout the several views of the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following description of the presently contemplated best mode ofpracticing the invention is not to be taken in a limiting sense, but ismade merely for the purpose of describing the general principles of theinvention. The scope of the invention should be determined withreference to the claims.

Referring to FIG. 1, an exemplary turbo code encoder is shown whereinone embodiment of a Turbo Code trellis termination design terminates oneencoder 10 (a first encoder) while disabling another encoder 10′ (asecond encoder) and at a different time terminates the other encoder 10′(second encoder) while disabling the encoder 10 (first encoder).

The encoders (first and second encoders) 10, 10′ of the turbo codeencoder of FIG. 1 are constituent encoders configured in a parallelconcatenation. It is well known in the art that a constituent encoderemploy a configuration of modular adders 17, 20, 26, 28, 30, 24, and 25,and shift registers 18, 21, 22, coupled through nodes (such as node 32)to produce output bits, including tail output bits, X(t), Y_(o)(t),Y₁(t), for example, depending upon the encoding scheme. FIG. 1 is justone example of such a parallel concatenation of constituent encoders,wherein an interleaver device (Interleaver) 16 is employed between aninput for X(t) and the second encoder 10′, and wherein additionally, apuncturer 36 is employed, switchably coupled to respective encoderoutputs for each of the encoders (first and second encoders) 10, 10′. Asdescribed herein, tail input bits will mean the bits X, and X′ in FIG.1, and tail output bits will mean the bits X, X′, Y_(o), Y_(o)′, Y₁ orY₁′. In other turbo encoders, there may be more than two constituentencoders. Each of the constituent encoders may utilize a fewer orgreater number of shift registers than in FIG. 1.

In FIG. 1, after message bits X(t) are encoded, a switch 12 is moved toa feedback position to allow the generation of three (3) consecutivetail input bits, in this example, generated from the contents of each ofthree shift registers 18, 21, and 22 (also referred to herein as a firstshift register 18, a second shift register 21, and a third shiftregister 22). In general, a number of tail input bits X(t), X′(t) forterminating a constituent encoder is equal to a number of shiftregisters in that encoder.

At the end of each clock cycle, new tail input bits X(t), X′(t) aregenerated for zeroing out each respective shift register of the threeshift registers, 18, 21 and 22.

In one embodiment of the invention the encoders 10, 10′ are terminatedsimultaneously within three clock cycles, each with its own tail inputbit X(t), X′(t). Alternatively, the first encoder 10 is first terminatedwhile the second encoder 10′ is disabled, followed by the second encoder10′ being terminated while the first encoder 10 is disabled.

In the variation with the encoders 10, 10′ terminated at different timesthe encoders 10, 10′ can be terminated in consecutive clock cycles,wherein six (6) consecutive clock cycle tail input bits X(t), X′(t),consecutively terminate both the encoders 10, 10′.

As can be seen from FIG. 1, a second tail input bit sequence 34′ forterminating the second encoder 10′ is fed back into the second encoder10′ through a switch 12′ and circuit 14′. Tail input bits X(t), X′(t)are not interleaved by the turbo interleaver 16. Similarly, a tail inputbit sequence 34 for terminating the first encoder 10 is fed back intothe first encoder 10 through another switch 12 and another circuit 14.

The zeroing of the shift registers 18, 21, 22, prior to implementing apuncturing scheme per an embodiment of the invention, is triggered by abeginning and an ending tail input bit sequence X(t), X′(t), eachsequence having a number n of tail input bits X(t), X′(t) equal to thenumber n of shift registers 18, 21, 22 or 18′, 21, 22 coupled to eachone of the encoders 10, 10′.

As with information and coded bits, tail output bits X, Y_(o), Y₁, X′,Y_(o), Y₁′ are also punctured by the puncturer 36.

Table 1 indicates associated tail output bit puncturing patterns havingindicator sequences (e.g., “111 000”) identifying which bits to punctureand which bits to transmit. The indicator sequence, comprising “1”'s or“0”'s is selected in accordance with an encoder rate. In this notation,“1” indicates the tail output bit should be transmitted and “0”indicates that the tail output should be punctured. Certain entries inTable 1 are labeled “repeat”, which means that transmitted bits aretransmitted twice.

The tail input bit sequences 34, 34′, which comprise tail input bits X,and X′, are generated after the encoders 10, 10′ encode the informationbits with the switches 12, 12′ (FIG. 1), while the switches 12, 12′ arein an up position. The first n/R tail output bits X₁, Y_(o), Y₁, whereinn is the number of shift registers 18, 21, 22 or 18′, 21′, 22′ perconstituent encoder (n=3 in FIG. 1), and wherein R is a turbo code ratebeing employed, are generated by clocking the first encoder 10 n timeswith its switch 12 in the down position while the second encoder 10′ isnot clocked, and puncturing or repeating the resulting tail output bitsX₁, Y_(o), Y₁, X′, Y_(o)′, Y₁′ according to Table 1 below. The last n/Rtail output bits X′,Y_(o)′,Y₁′ are generated by clocking the secondencoder 10′ n timer with its switch 12′ in the down position while thefirst encoder 10 is not clocked, and puncturing or repeating theresulting tail output bits according to Table 1. These final output bitsare denoted by X′, Y_(o)′ or Y₁′.

For rate ½ turbo codes, the tail output bits for each of a first n tailinput bit (also referred to herein as “the beginning tail bit sequenceX(t)”) are XY₀, and the tail output bits for each of a last n tail bitperiods (also referred to herein as “the ending tail bit sequenceX′(t)”) are X′Y₀′. For rate ⅓ turbo codes, the tail output bits for eachof the first n tail input bits are XXY₀, and the tail output bits foreach of the last n tail bits are X′X′Y₀′. For a rate ¼ turbo code, thetail output bits for each of the first n tail input bits are XXY₀Y₁ andthe tail output bits for each of the last n tail input bits periods areX′X′Y₀′Y₁′.

Tail inputs bits are not interleaved by the interleaver 16. They areadded after the encoding of the information bits. TABLE 1 PuncturingPatterns for Tail Output Bits Rate 1/2 1/3 1/4 X(t) 111 000 111 000 111000 Repeat Repeat Y₀(t) 111 000 111 000 111 000 Y₁(t) 000 000 000 000111 000 X′(t) 000 111 000 111 000 111 Repeat Repeat Y₀′(t) 000 111 000111 000 111 Y₁′(t) 000 000 000 000 000 111

When employing Table 1 to design puncturing patterns for tail outputbits, the row designation “Repeat” means that for a rate ⅓ or a rate ¼turbo code, when transmitted, the bits X and X′ are transmitted twice.

For a rate ½ turbo code, the puncturing table is read first from top tobottom, and then from left to right. For a rate ⅓ turbo code and a rate¼ turbo code, the puncturing table is read first from top to bottom,repeating X(t) and X′(t), and then from left to right.

The puncturing patterns in Table 1 are chosen so that:

(1) A number of transmitted tail output bits during trellis terminationis 1/R for each trellis branch wherein R is the turbo code rate employedduring information bit transmission. Advantageously, this conditionensures that the same turbo code rate is used for trellis termination asfor information bit transmission.

(2) Only output branches of the encoders 10, 10′ used during informationbit transmission are used for trellis termination. For example, for rate½ and rate ⅓ turbo coders, only X(t), X′(t), Y₀(t) and Y′₀(t) aretransmitted during information bit transmission; Y₁(t) and Y′₁(t) arealways punctured. Therefore, only X(t), X′(t), Y₀(t) and Y′₀(t) aretransmitted during the trellis termination stage, as well.Advantageously, therefore, if a manufacturer only wanted to implement arate ½ and encoder, such a manufacturer would only have to implementtransmissions of bits from branches X, Y₀ or X′, Y₀′.

(3) In order to meet conditions (1) and (2), it may require repetitionof some tail output bits during trellis termination. That is, to bothkeep the turbo code rate the same, and to only use output branches usedin information bit transmission, it may be necessary to repeat one ormore of the tail bits for each encoder 10, 10′ in order to keep theturbo code rate the same.

In the preferred embodiment illustrated by Table 1, X(t) and X′(t) areselected to be repeated in both the turbo code rate ⅓ and rate ¼ cases.Table 1 may also be employed irrespective of whether the encoders 10,10′ are terminated concurrently or non-concurrently.

Alternative embodiments are envisioned, in keeping within the spirit ofthe invention wherein another tail output bit is selected to berepeated, such as, for example that corresponding to Y₀(t) and Y₀′(t).

Alternatively, where a code rate lower than ¼ is employed it may benecessary to repeat more than one tail output bit per encoder 10, 10′,in which case an additional tail bit besides X(t) may be repeated, suchas repeating X(t) and Y₀(t) or repeating X(t) twice or any combinationwhatsoever.

While the invention herein disclosed has been described by means ofspecific embodiments and applications thereof, numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the scope of the invention set forth in the claims.

1-7. (canceled)
 8. A communication system for use in wirelesscommunication, the communication system comprising: an encodercomprising shift registers and encoding information bits to generateoutput bits at at least four output branches and to selectively transmitencoded bits from the at least four output branches according to atleast one code rate (R); and an interleaver connected to the encoder andinterleaving the information bits, wherein the tail output bits aredetermined based on shift register state values of the encoder.
 9. Thecommunication system of claim 8, wherein (R) comprises a plurality ofcode rates.
 10. The communication system of claim 8, wherein the tailoutput bits are based on last shift register state values after encodingall of the information bits.
 11. The communication system of claim 8,wherein the tail output bits are determined in response to the code rate(R).
 12. The communication system of claim 8, wherein the tail outputbits are selectively transmitted based on the code rate (R).
 13. Thecommunication system of claim 8, wherein the output bits furthercomprise a fifth output branch and a sixth output branch.
 14. Thecommunication system of claim 8, wherein the output bits from the thirdoutput branch and the fourth output branch are from interleavedinformation bits.
 15. The communication system of claim 8, wherein thecode rate (R) comprises one of ½, ⅓ and ¼.
 16. The communication systemof claim 8, wherein the tail output bits are pre-determined inassociation with the shift register state values.